Thermal management using endothermic heat sink

ABSTRACT

A thermal management system includes a slurry generator, an injector pump coupled to the slurry generator, a heat exchanger reactor coupled to the injector pump, wherein the heat exchanger reactor is adapted to subject a thermally expendable heat absorption material to a temperature above 60° C. and a pressure below 3 kPa, and wherein the expendable heat absorption material endothermically decomposes into a gaseous by-product. A vapor cycle system is coupled to the heat exchanger reactor and is operatively connected to a thermal load. A thermal energy storage system may be coupled to the vapor cycle system and the thermal load. The thermal energy storage system may isolate the heat exchanger reactor from thermal load transients of the thermal load.

GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States for all government purposes withoutthe payment of any royalty.

BACKGROUND Field of the Invention

The embodiments herein generally relate to thermal management systems,and more particularly to using ammonium carbamate for thermal managementsystems.

Background of the Invention

High-performance aircraft are experiencing increased use of high-powerelectronics. Many of these electronic systems produce high-flux,low-grade heat that must be removed to avoid component failure. Thedissipation of high-flux, low-grade heat from an aircraft presentsunique challenges. High-flux loads generally require large,high-capacity cooling systems. Low-grade heat rejection typicallyrequires large heat dissipation surfaces, which may not be readilyintegrated into airframes. New composite materials which have lowthermal conductivities are being used for airframes and present furtherthermal management challenges. In addition, an aircraft's operatingenvelope may preclude dissipating low-grade heat to the atmosphere dueto high aircraft skin and stagnation air temperatures during high-speedflight. Jet fuel has been used to absorb heat from aircraft systemsbefore combustion. However, jet fuel may degrade in the presence of heatand dissolved O₂ may form surface deposits and block fuel passages.Furthermore, the available heat sink decreases in time as the jet fuelis consumed. Thus, the use of jet fuel for thermal management is notwithout limitations, and alternative techniques to dissipate the heatare required. Accordingly, the issue of managing low-quality waste heatonboard modern high-performance aircraft represents a criticalperformance limitation that can severely restrict flight envelopes,preclude the integration of high-power electronics, or compromise systemreliability.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, an embodiment herein provides a thermalmanagement system comprising a slurry generator; an injector pumpcoupled to the slurry generator; a heat exchanger reactor coupled to theinjector pump, wherein the heat exchanger reactor is adapted to subjecta thermally expendable heat absorption material to a temperature above60° C. and a pressure below 3 kPa, and wherein the expendable heatabsorption material endothermically decomposes into a gaseousby-product; and a vapor cycle system coupled to the heat exchangerreactor and operatively connected to a thermal load.

The system may comprise a thermal energy storage system coupled to thevapor cycle system and the thermal load. The thermal energy storagesystem may isolate the heat exchanger reactor from thermal loadtransients of the thermal load. The heat exchanger reactor may receive apulsed heat load of specified heating rates from the thermal load. Theexpendable heat absorption material may comprise an ammoniated salt. Theexpendable heat absorption material may comprise any of ammoniumcarbamate and ammonium bicarbonate. The system may comprise a controlleroperatively connected to the vapor cycle system to control a temperatureof the thermal load being applied to the vapor cycle system toapproximately an ambient temperature surrounding the thermal load.

Another embodiment provides a heat sink comprising a heat exchangerreactor to permit an expendable heat absorption material to absorb heatfrom a thermal load at a temperature above 60° C. and a pressure below 3kPa, wherein the expendable heat absorption material endothermicallydecomposes; a vapor cycle system coupled to the heat exchanger reactorand configured to transfer heat from the thermal load; a firstcontroller to regulate a heat absorption into the vapor cycle system;and a second controller to regulate an amount of the expendable heatabsorption material into the heat exchanger reactor. The heat sink maycomprise a heat capacitor to isolate the heat exchanger reactor fromthermal transients of the thermal load. The expendable heat absorptionmaterial may comprise an ammoniated salt. The heat sink may comprise acarrier fluid mixed with the expendable heat absorption material,wherein the carrier fluid comprises any of ethylene glycol and propyleneglycol.

Another embodiment provides a method for managing heat transfer of athermal load, the method comprising providing a thermal managementsystem comprising a heat exchanger reactor containing an expendable heatabsorption material; applying heat from a thermal load to the thermalmanagement system at a temperature between approximately 18-30° C. usinga thermal load coolant; transferring heat from the thermal load to atransfer fluid at a temperature above approximately 60° C. creating aheated transfer fluid, wherein the heated transfer fluid is introducedinto the heat exchanger reactor; transferring heat from the heatedtransfer fluid into the expendable heat absorption material, wherein theexpendable heat absorption material endothermically decomposes into agaseous by-product; and maintaining the thermal load coolant within aspecified temperature range independent of heat load dynamics.

The method may comprise applying the heat from the thermal load at asteady rate, or the method may comprise applying the heat from thethermal load at an unsteady rate. The method may comprise calculating anamount of heat transfer from the thermal load to the expendable heatabsorption material. The expendable heat absorption material maycomprise an ammoniated salt. The expendable heat absorption material maycomprise any of ammonium carbamate and ammonium bicarbonate. The methodmay comprise suspending the expendable heat absorption material in acarrier fluid comprising any of ethylene glycol and propylene glycol.The method may comprise isolating the heat exchanger reactor fromthermal transients of the thermal load. The method may comprisecontrolling a temperature of the thermal load to approximately anambient temperature surrounding the thermal load.

These and other aspects of the embodiments herein will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following descriptions, while indicatingpreferred embodiments and numerous specific details thereof, are givenby way of illustration and not of limitation. Many changes andmodifications may be made within the scope of the embodiments hereinwithout departing from the spirit thereof, and the embodiments hereininclude all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, in which:

FIG. 1 is a block diagram illustrating a thermal management system usedfor a steady-state thermal load, according to an embodiment herein;

FIG. 2 is a block diagram illustrating a thermal management systemintegrated with a thermal energy storage system used for a dynamicthermal load, according to an embodiment herein;

FIG. 3 is a graphical illustration of an ammonium carbamate (AC)decomposition curve;

FIG. 4 is a block diagram illustrating an apparatus used for asteady-state thermal load, according to an embodiment herein;

FIG. 5 is a block diagram illustrating an apparatus including a thermalmanagement system used for a dynamic thermal load, according to anembodiment herein;

FIG. 6 is a graphical illustration depicting results for a heatexchanger (HEX) reactor temperature and pressure, coolant temperature,and AC feed histories during an uncontrolled-capacity preliminaryexperiment, according to an embodiment herein;

FIG. 7 is a graphical illustration depicting results for a HEX reactorand process fluid temperatures for the reduced-capacity experiment,according to an embodiment herein;

FIG. 8 is a graphical illustration depicting results for a temperaturehistory of a HEX reactor transfer loop, a HEX reactor slurry loop, andan evaporator coolant loop for a transient heat load experiment with athermal energy storage system, according to an embodiment herein;

FIG. 9 is a graphical illustration depicting representative temperatureresponses to pulsed heat input during a TES-integrated dynamic heat loadexperiment, according to an embodiment herein;

FIG. 10 is a system block diagram illustrating a heat sink, according toan embodiment herein; and

FIG. 11 is a flow diagram illustrating a method for managing heattransfer of a thermal load, according to an embodiment herein.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the disclosed invention, its various features and theadvantageous details thereof, are explained more fully with reference tothe non-limiting embodiments that are illustrated in the accompanyingdrawings and detailed in the following description. Descriptions ofwell-known components and processing techniques are omitted to notunnecessarily obscure what is being disclosed. Examples may be providedand when so provided are intended merely to facilitate an understandingof the ways in which the invention may be practiced and to furtherenable those of skill in the art to practice its various embodiments.Accordingly, examples should not be construed as limiting the scope ofwhat is disclosed and otherwise claimed.

In the drawings, the size and relative sizes of layers and regions maybe exaggerated for clarity. The principal engineering challenges to thethermal systems engineer are the large quantities of heat involved, low(and carefully controlled) coolant temperatures required, and thelimited availability of heat sinks to which the waste heat mustultimately be rejected. The use of endothermic chemical reactions toserve as controllable, flight envelope-independent heat sinks hasconsiderable potential in addressing the latter of the aforementionedengineering challenges, but must be integrated as part of asystems-level approach in order to address all three: acquisition,control, and rejection. The embodiments herein provide an expendablethermal management system (ETMS) architecture integrating a vaporcompression heat pump, phase change thermal energy storage, and heatexchanger-reactor (HEX reactor) to utilize the endothermic decompositionof ammonium carbamate as an expendable heat sink. Referring now to thedrawings, and more particularly to FIGS. 1 through 11, where similarreference characters denote corresponding features consistentlythroughout, there are shown exemplary embodiments.

FIG. 1 illustrates a thermal management system 10 comprising a slurrygenerator 15, an injector pump 20 coupled to the slurry generator 15,and a heat exchanger (HEX) reactor 25 coupled to the injector pump 20.The HEX reactor 25 is adapted to subject a thermally expendable heatabsorption material 30 to a temperature above 60° C. and a pressurebelow 3 kPa. The expendable heat absorption material 30 endothermicallydecomposes into a gaseous by-product 35. In an example, the expendableheat absorption material 30 comprises an ammoniated salt such asammonium carbamate. More particularly, the ammonium carbamate may have adecomposition temperature of 60° C. and enthalpy of decomposition of2.01 MJ/kg in one example, and a decomposition temperature of 85° C. andenthalpy of decomposition of 1.97 MJ/kg in another example. Moreover,another example of the expendable heat absorption material 30 includesammonium bicarbonate having a decomposition temperature of 85° C. and anenthalpy of decomposition of 2.12 MJ/kg. The system further comprises avapor cycle system 40 coupled to the HEX reactor 25 and operativelyconnected to a thermal load 45.

As shown in FIG. 2, with reference to FIG. 1, the system 10 may comprisea thermal energy storage system 50 coupled to the vapor cycle system 40and the thermal load 45. The thermal energy storage system 50 mayisolate the heat exchanger reactor 25 from thermal load transients ofthe thermal load 45. The HEX reactor 25 may receive a pulsed heat loadof specified heating rates from the thermal load 45. The expendable heatabsorption material 30 may comprise an ammoniated salt. The expendableheat absorption material 30 may comprise any of ammonium carbamate andammonium bicarbonate. The system 10 may comprise a controller 55operatively connected to the vapor cycle system 40 to control atemperature of the thermal load 45 being applied to the vapor cyclesystem 40 to approximately an ambient temperature surrounding thethermal load 45. In an example, the controller 55 may be any type ofprocessor, microcontroller, application specific processor, applicationspecific integrated circuit, or digital signal processor.

The cooling technique provided by the embodiments herein utilizes anexpendable heat absorption material 30 that absorbs heat and then isexpelled. The material 30 undergoes an endothermic chemical reaction.Endothermic chemical reactions may involve greater changes in enthalpyrelative to sensible heating or thermophysical phase change. An exampleof the material 30 that undergoes endothermic decomposition with highenthalpy change is ammonium carbamate (AC), which is a solid ammoniumsalt that decomposes into carbon dioxide and ammonia gases in thetemperature range of 10-60° C.:

[NH₄][H₂NCOO]⇄CO₂+2NH₃   (1)

The enthalpy of decomposition for AC is approximately 2 MJ/kg, whichcompares well with water vaporization at atmospheric pressure. FIG. 3shows solid-gas decomposition pressures at different temperatures forAC. In an example, ammonium carbonate can slowly decompose at standardtemperature (e.g., 0° C.) and pressure (e.g., 100 kPa). There is a widerange of temperatures and pressures over which decomposition can occur.For example, FIG. 3 shows that the solid material tends to decomposeinto a gaseous mixture when heated above 60° C. at 100 kPa, or when thepressure is reduced below 10 kPa at 20° C. To facilitate heat and masstransfer, AC can be suspended in a carrier fluid. According to anexample, AC can be suspended in ethylene glycol or propylene glycol (PG)and such that, decomposing in a batch reactor under reduced pressure,heat from the thermal load 45 will be absorbed. A chevron plate heatexchanger (CPHE) may be utilized as the HEX reactor 25 to enablecontinuous heat absorption via AC decomposition. HEX reactor 25 is apromising means of realizing the thermal management potential of ACbecause it can offer a high volumetric heat transfer capacity to effecthigh reaction yields

Continuous heat absorption in a HEX reactor 25 may be achieved bysubjecting a slurry of AC suspended in propylene glycol (PG) to elevatedtemperatures under atmospheric and reduced pressures. The rate of heatrejection is sensitive to the reaction temperature (i.e., thetemperature at which the AC/PG mixture decomposes), and operation underreduced pressure allows the process to achieve the objective of near100% AC decomposition into the gases described above. The HEX reactor 25in the thermal management system 10 utilizes integration with amechanism to control reaction temperature, and an assessment of the HEXreactor's 25 response to time variant heat loads. Accordingly, theembodiments herein provide for the endothermic decomposition of AC in aHEX reactor 25 as part of a thermal management system 10 to control thetemperature of the thermal load 45. In an example, the mixture of the ACand PG is subjected to elevated temperatures (above 60° C.) and reducedpressures (below 3 kPa) for at least one minute.

Experiment

Two experimental configurations were used to investigate either steady(FIG. 4) or dynamic (FIG. 5) heat loads 45. FIGS. 4 and 5, withreference to FIGS. 1 through 3, show the experimental apparatus 5 a, 5 bfor the two load conditions, steady and dynamic, which can be dividedinto subsystems 10, 46. As illustrated in FIGS. 4 and 5 and describedbelow the various components and parameters are merely provided asexperimental examples, and the embodiments herein are not restricted tothese particular components, configurations, or parameters. Thesubsystem 10 depicted on the left side of FIGS. 4 and 5 is the ETMS 10comprising the HEX reactor 25, slurry generator 15, vacuum pump 21, andliquid/gas separator 22. The subsystem 46 on the right side is theimposed heat load 45 which has the capability to provide either steadyor dynamic heat loads. A third subsystem 56 (shown in FIG. 5) interposedbetween the ETMS 10 and the imposed load 45 is the thermal energystorage (TES) system 50 which acts as a thermal capacitance in theapparatus 5 b.

The HEX reactor 25 generally comprises a reactant side 11 and a transferside 12. In one example, the HEX reactor 25 may comprise plate heatexchangers, which may provide enhanced mixing and high surfacearea-to-volume ratios, which enable high thermal power densities. Someexamples of the configuration of the HEX reactor 25, which may beutilized, are described in Niedbalski, N., et al., “Study of amulti-phase hybrid heat exchanger-reactor (HEX reactor): PartI—Experimental characterization,” International Journal of Heat and MassTransfer 70 (2014) 1078-1085; Niedbalski, N., et al., “Study of amulti-phase hybrid heat exchanger-reactor (HEX reactor): PartII—Numerical prediction of thermal performance,” International Journalof Heat and Mass Transfer 70 (2014) 1086-1094; and Johnson, D., et al.,“An Integrated Chemical Reactor-heat Exchanger based on AmmoniumCarbamate,” Proceedings of the SAE 2012 Power Systems Conference, Oct.22, 2012, the complete disclosures of which, in their entireties, areherein incorporated by reference. FIGS. 4 and 5 show that the HEXreactor 25 is operatively connected to the slurry generator 15 and thevapor cycle system 40.

The slurry generator 15 may comprise a gravimetric feeder 16, a mixer17, and a charge pump 18. The mixer 17 connects to the slurry injectorpump 20. The gravimetric powder feeder 16 meters the expendable heatabsorption material 30, which may be dry AC powder (hereinafter referredto as “AC”), into the mixer 17, which suspends the AC in a carrier fluid85, which is metered into the mixer 17 using the charge pump 20. A flowmeter 19 may be used to control the flow of the carrier fluid 85 intothe mixer 17. An example of the carrier fluid 85, which may be used, is≥99.5% grade PG available from Sigma-Aldrich Corporation, Missouri, USA.The carrier fluid 85, which is a heat transfer fluid, and PG may beutilized since it is non-toxic and enhances the decomposition raterelative to pure AC powder in air. In an example, the carrier fluid 85used to generate the slurry may initially be provided at roomtemperature. The AC/PG slurries are generated such that the AC feed rateand PG flow rate may be adjusted to accommodate a desired ACconcentration. In an example, the slurry flow rate range may beapproximately 0.5-0.8 L/min, with AC/PG ratios of approximately 240-360g of AC per L of PG.

The AC/PG slurry is pumped into the HEX reactor 25 via the slurryinjector pump 20. Varying the PG flow rate of the slurry generator 15also varies the reactor residence time, which is the amount of time theAC/PG slurry spends in the HEX reactor 25. A liquid-gas separator 22removes undissolved waste gases 35 from the PG. The PG containingdissolved waste products (e.g., waste gases 35) and unreacted AC ispumped into a waste container 23.

As shown in FIGS. 4 and 5, a vacuum pump 21 may be provided downstreamof the liquid-gas separator 22 to reach a desired pressure in thereactant side 11 of the HEX reactor 25. The HEX reactor 25 pressure doesnot have to be actively controlled, according to an example. Rather, theresulting pressure in the HEX reactor 25 may be a consequence of thecapacity of the vacuum pump 21 and the volume of undissolved gasproduced in the HEX reactor 25. The pressure in the slurry line upstreamof the HEX reactor 25 may be maintained above the decomposition pressurefor a given temperature (as indicated in FIG. 3) to inhibitdecomposition before the slurry enters the HEX reactor 25. This may beachieved by the presence of a small inlet orifice 24 at the inlet 8 ofthe HEX reactor 25. In an example, the inlet orifice 24 may beapproximately 1.6 mm, although other configurations are possible. Theslurry injector pump 20 acts as an additional pressure isolator betweenthe HEX reactor 25 and the mixer 17, which prevents prematuredecomposition in the slurry.

The system 10 may be operated in an open-loop manner such that there isno feedback of the output of the system 10. In an example, the carrierfluid 85, such as PG, may be supplied for slurry generation, pumpedthrough the HEX reactor 25, and then removed from the system 10 afterthe AC is reacted. To remove waste material, such as gas 35, from thesystem 10, a vacuum-rated scavenge sump 21 may be provided to collect PGfrom the separator 22. Pressure in the scavenge sump 21 may be cycledbetween sub-ambient and ambient to permit PG removal.

The vapor-cycle system (VCS) 40 may be configured as a heat pump withassociated controls. The VCS 40 is used to increase the temperature(e.g., approximately 16-18° C.) at which heat is absorbed from thethermal load coolant 75 to the reaction temperature (e.g., approximately55-80° C.). The VCS 40 delivers the absorbed heat to the HEX reactor 25through a water transfer loop 27 comprising an inlet 7 of the watertransfer loop 27 to the VCS 40 from the HEX reactor 25, and an outlet 6of the water transfer loop from the VCS 40 to the HEX reactor 25. Thewater in the transfer loop 27 may flow at the same rate as the thermalload coolant 75.

The thermal load 45 to be absorbed by the system 10 may be generated bythe subsystem 46. The subsystem 46 may produce either steady-state orpulsed thermal loads 45. In an example, steady-state thermal loads 45may be produced by preconditioning a water supply to a desiredtemperature in the range of approximately 15-60° C. The preconditionedwater may then be pumped through an evaporator 42 in the VCS 40

The water may be heated by an inline heater 48. The power of the heater48 may be controlled by a pulse width modulated (PWM) power supply 59,which may be programmed to produce power pulses of adjustable amplitude,duration, and duty cycle. The subsystem 46 may be controlled by asystem-design software platform (not shown), such as the LabVIEW®program available from National Instruments Corporation, Texas, USA,which allows a desired supply temperature and heat pulse profile to beprovided. A heater timer signal 57 may be sent from the subsystem 46 toa data acquisition component 58 in the system 10 to perform dataanalysis.

As provided in FIG. 5, the TES 50 is configured between the subsystem 46and the system 10. The TES 50 may utilize a phase change process tostore thermal energy. The TES 50 may have a phase-change temperature ofapproximately 18° C. and a water circulation pump 51. The TES 50 may beused as a thermal capacitance to isolate the HEX reactor 25 from heatload transients. The high temperature side 52 of the TES 50 isoperatively connected to the subsystem 46, and the low temperature side53 is operatively connected to the system 10. Additional instrumentation(not shown) may be provided such as thermocouples in the water ports anda flow meter in the subsystem 46 and TES 50.

The system 10 responds to temperature changes in the load source coolant75 by independently modulating the heat pump capacity and sinkconditions of VCS 40 (i.e., HEX reactor 25 slurry flow). In an example,the sequencing of the various components in the apparatus 5 a, 5 b isfully automated. The VCS heat pump process may be controlled bymaintaining a constant superheat and inlet temperature of the evaporator42. A hot gas bypass (HGBP) valve (not shown) may be utilized tomaintain the inlet temperature of the evaporator 42. Control of thesetwo variables allows the system 10 to adapt to both the magnitude andtemperature of the thermal load 45. The HEX reactor 25 (e.g., sink)conditions may be controlled by specifying a reaction temperature range.The range may set the “on” and “off” reaction temperature setpoints, andwhen the reactant outlet temperature exceeds or falls below therespective setpoint, the slurry generator 15 and HEX reactor 25 areactivated/deactivated, thereby starting/ceasing AC decomposition. Theslurry generation parameters include the AC feed rate and PG flow rate,which are fixed to deliver a slurry of constant AC concentration whenthe slurry generator 15 and HEX reactor 25 are active. In an example ofthe experiment, the AC feed rate and PG mass flow rate may be set toapproximately 1.8 g/s and 13 g/s, respectively. This particular AC feedrate/PG flow rate combination may deliver up to approximately 4500 W ofcooling capacity assuming a decomposition enthalpy of 1800 J/g. The PGcarrier fluid 85 provides an additional approximately 35 W/° C. coolingcapacity.

At the interface of the system 10 and thermal load 45, the burden on thesystem 10 is set by the load source heat input (e.g., up toapproximately 4 kW) and the coolant inlet temperature, while the targetcoolant outlet temperature serves as the constraint condition. Thecoolant inlet temperature may be varied between approximately 15° C. to24° C. at a fixed flow rate of approximately 10 L/min, while the targetcoolant temperature may be set between approximately 10° C. and 18° C.with a maximum permissible error of ±2° C. Temperatures, pressures, andflow rates may then be measured. For example, thermocouples (not shown)may be positioned on ports (not shown) of the HEX reactor 25, the TES50, and the load source coolant tank outlet (e.g., coolant supply 47).Pressure transducers (not shown) may be installed in the ports (notshown) of the reactant side 11 of the HEX reactor 25 and the inlet 61 tothe vacuum pump 21. One or more turbine-type flow meters 19, 44, 49, 54may be provided in the PG supply line, in the transfer loop, in thecoolant loop, and in the TES 50 recharge loop, respectively. To ensureviability of the data, experimentally, all instruments are calibrated toprovide values with less than 1% error. Table 1 lists example instrumentranges and uncertainties used in the experiment, however the embodimentsherein are not restricted to these particular parameters.

TABLE 1 Instrument accuracies and ranges Instrument Accuracy RangeMfg./Model Pressure transducers ±0.76% FS 0-445 kPA Omega ® PX409-050A5V Thermocouples ±0.1° C. 5-105° C. Omega ® KMQSS- 062U-6 Transferloop flow meters (2) ±0.1 L/min 0.5-15 L/min Omega ® FTB1412 Load sourcecoolant flow ±0.1 L/min 0.5-30 L/min Omega ® meter FTB1412 Propyleneglycol flow meter ±0.1 L/min 0.15-3.5 L/min Omega ® FTB205

Experiments were conducted to demonstrate the cooling capacity andtemperature regulation capability of the system 10. Both steady-state(heat load not varying over the experiment) and dynamic heat loadexperiments were performed. For steady-state experiments, the system 10was exposed to a steady flow of coolant 75 at a constant temperature foran extended period; approximately 5 min. The intent of these experimentswas to demonstrate correct operation of the system 10, and todemonstrate that the cooling capacity of the system 10 could becontrolled. Two subcases were considered in the experiment:

-   -   1. Steady-state heat transfer study subjected to a constant        thermal load 45, without provisions to control the VCS capacity.    -   2. Steady-state heat transfer study subjected to a constant        thermal load 45, with VCS capacity matched to the imposed heat        load using a hot gas bypass method.

Dynamic heat load experiments subjected the system 10 to pulsed heatloads of specified heating rates. The purpose of these experiments wasto demonstrate that the system 10 could maintain the coolant 75 within aspecified temperature range, independent of heat load dynamics. Theseexperiments were performed both without TES 50 (e.g., using theapparatus 5 a in FIG. 4) and with TES 50 between the subsystem 46 andthe VCS 40 (e.g., using the apparatus 5 b in FIG. 5). All experimentswere conducted and their respective adjustable parameters weretabulated, as provided in Table 2.

TABLE 2 Test matrix showing test designation and corresponding imposedcoolant temperature, coolant outlet temperature constraint, and loadsource input. For the dynamic heat load experiments, the heat loadreported is the peak load amplitude. Coolant Coolant Heat load IN (° C.)OUT (° C.) (kW) Test description 21.0 13.0 4.5 Steady Load, UncontrolledCapacity (Cap.) 21.0 13.0 4.5 Steady Load, Uncontrolled Cap. 21.0 13.04.5 Steady Load, Uncontrolled Cap., reduced superheat 21.0 15.5 4.0Steady Load, Uncontrolled Cap. 18.0 12.5 4.0 Steady Load, UncontrolledCap. 17.0 14.0 2.0 Steady Load, Uncontrolled Cap. 21.0 18.0 4.0 DynamicLoad, No TES 21.0 18.0 4.0 Dynamic Load, TES integrated

In general, the experimental procedure included a “preconditioning”phase during which the HEX reactor 25 was heated to the desired reactiontemperature in the absence of AC, followed by a “decomposition” phasewhere the slurry flow from the slurry generator 15 was initiated tobegin AC decomposition. The preconditioning phase served to mitigate thecontribution of thermal capacitance, which may be due to steel faceplates on the HEX reactor 25, to the energy balance analysis and ensuredconsistent initial conditions at the desired reaction temperature foreach experiment. For the steady-state experiments, preconditioning wasaccomplished by using the VCS 40 to transfer thermal energy from theload coolant 75, which was preheated to the desired coolant inlettemperature, as shown in Table 2, to the HEX reactor 25 withoutabsorbing heat via AC decomposition. The HEX reactor transfer loop wascirculated continuously during this period. Once the HEX reactor 25setpoint temperature was attained, the gravimetric feeder 16 and slurryinjector pump 20 began generating and introducing the slurry into theHEX reactor 25 to begin the decomposition phase. The slurry loopcontinued to run until the AC supply in the gravimetric feeder 16 wasdepleted, marking the conclusion of the experiment.

The heat absorption/cooling rate by the system 10 may be determined bymeasuring the coolant temperature reduction. To generate the control setpoint for a controller 55 (shown in FIG. 2), the target coolant outlettemperature was calculated based on the imposed coolant inlettemperature and desired heat absorption rate:

$\begin{matrix}{T_{C,{out}} = {T_{C,{in}} - \frac{q_{C}}{{\overset{.}{m}}_{c}c_{p}}}} & (2)\end{matrix}$

where T_(C,out) is the coolant outlet temperature, T_(C,in) the coolantinlet temperature, q_(C) is the rate of heat absorption from thecoolant, {dot over (m)} is the mass flow rate, and c_(P) is theconstant-pressure specific heat capacity of the coolant at the averagebulk temperature. For the uncontrolled capacity experiments, the coolantinlet and outlet temperatures were set at approximately 21° C. and 13°C., respectively, to permit the system 10 to achieve its maximum heatrejection capacity.

The amount of thermal energy absorbed by the endothermic reaction wascalculated from an energy balance analysis on the HEX reactor 25.Neglecting the heat transferred to the HEX reactor 25 itself, the heatabsorbed by endothermic decomposition may be estimated by comparing thesensible energy changes in the slurry and transfer fluid streams in theHEX reactor 25:

q _(decomp) =q _(HEX,Tx) −q _(HEX,S)   (3)

where q_(HEX,Tx) and q_(HEX,S) are the rates of sensible energy change,respectively, of the transfer loop fluid and slurry occurring in the HEXreactor 25, and q_(decomp) is the rate of heat consumption by thechemical reaction. In general, the energy balance for the reactant side11 of the HEX reactor 25 is:

$\begin{matrix}{q_{{HEX},{Tx}} = {{\int_{{HEX},{Tx}}{\frac{\partial T}{\partial t}\rho \; c_{p}{dV}}} + \left\lbrack {{\overset{.}{m}}_{c}{c_{p}\left( {T_{in} - T_{out}} \right)}} \right\rbrack_{{HEX},{Tx}}}} & (4) \\{q_{{HEX},S} = {{\int_{{HEX},S}{\frac{\partial T}{\partial t}\rho \; c_{p}{dV}}} + \left\lbrack {{\overset{.}{m}}_{c}{c_{p}\left( {T_{in} - T_{out}} \right)}} \right\rbrack_{{HEX},S}}} & (5)\end{matrix}$

The inlet and outlet temperature histories for the HEX reactor 25obtained from the experiments consistently showed that the slopes forT_(in) and T_(out) were nearly equal. Hence, it was assumed that ∂T/∂tdoes not vary appreciably over the length of the HEX reactor 25, and theintegral terms on the right-hand side of Eqs. (4) and (5) may beapproximated as:

$\begin{matrix}{{\int_{{HEX},{Tx}}{\frac{\partial T}{\partial t}\rho \; c_{p}{dV}}} \approx {V_{{HEX},{Tx}}\left\lbrack {\rho \; c_{p}\frac{{dT}_{in}}{dT}} \right\rbrack}_{{HEX},{Tx}}} & (6) \\{{\int_{{HEX},{Tx}}{\frac{\partial T}{\partial t}\rho \; c_{p}{dV}}} \approx {V_{{HEX},S}\left\lbrack {\rho \; c_{p}\frac{{dT}_{in}}{dT}} \right\rbrack}_{{HEX},S}} & (7)\end{matrix}$

-   -   where V is the volume of the transfer loop or slurry side of the        HEX reactor 25 as indicated by the subscript. For most of the        experiments conducted, the steady-state approximation could be        invoked to reduce Eqs. (6) and (7) to:

q _(HEX,Tx) =[{dot over (m)} _(c) c _(p)(T _(in) −T _(out))]_(HEX,Tx)  (8)

q _(HEX,S) =[{dot over (m)} _(c) c _(p)(T _(in) −T _(out))]_(HEX,S)  (9)

For the experiments with dynamic heat loads and TES 50 incorporatedbetween the system 10 and the thermal load 45, this simplification wasapplied to the energy balance analysis on account of the significantthermal capacitance of the TES 50 compared to that of the preconditionedHEX reactor 25 and VCS 40. The steady-state experiments with constantheat load showed that the preconditioned HEX reactor 25, which was basedon the behavior of the slurry outlet temperature following the start ofthe AC decomposition, had first-order time constants of approximately20-30 s, whereas the charge/discharge time for the TES 50 wasapproximately 5 min. The considerable difference in timescales permitsthe assumption that the HEX reactor 25 and VCS 40 experience only asmall departure from steady state due to the transientrecharging/discharging of the TES 50. In fact, it was only necessary toinclude the transient term in the energy balance for the steady heatload experiment with reduced capacity, as further described below.

As an additional thermal performance metric to be considered alongsidethe capacity and stability of the system 10, the coefficient ofperformance (COP) was utilized. The COP was computed as follows:

$\begin{matrix}{{COP} = \frac{q_{{HEX},{Tx}}}{{\overset{.}{W}}_{comp}}} & (10)\end{matrix}$

where {dot over (W)}_(comp) is the energy input from the VCS compressor43 (including irreversibilities). Here, {dot over (W)}_(comp) wasestimated by a steady state energy balance on the transfer loop assumingno losses to the environment or thermal capacitances:

{dot over (W)} _(comp) ≈q _(HEX,Tx) −q _(evap,Tx)   (11)

where q_(evap,Tx) is the heat transferred into the evaporator 42 fromthe coolant supply 47 or the TES 50. In effect, the COP is a measure ofthe thermal efficiency penalty for affecting a temperature lift. Thismanifests as additional heat that must be conveyed to the sink (i.e., ACdecomposition).

To supplement the COP as a system performance measure, the effectivenessof the HEX reactor 25 was quantified by the conversion fraction; i.e.,the ratio of AC mass fed to AC mass reacted. The conversion fraction, c,under steady conditions is given by:

$\begin{matrix}{ɛ = \frac{{\overset{.}{m}}_{AC}}{q_{decomp}\text{/}\Delta \; H}} & (12)\end{matrix}$

where {dot over (m)}_(AC) is the feed rate of solid AC and ΔH is theheat of reaction for AC decomposition, which was assumed to beapproximately 1800 kJ/kg. The effect of this assumption on the reportedresults is described below.

In the experiment, uncertainty propagation was estimated with thewell-established Kline-McClintock method. Uncertainties associated withq_(HEX,Tx), q_(HEX,S), and q_(evap,Tx) were approximately 2% based onthe thermocouple and flow meter uncertainties tabulated in Table 1, andthe constant transfer loop flow rate used throughout each experiment.The uncertainty in the calculated value of {dot over (W)}_(comp) isgiven by:

$\begin{matrix}{\frac{\delta \; {\overset{.}{W}}_{comp}}{{\overset{.}{W}}_{comp}} = \sqrt{{\left( \frac{q_{{HEX},{Tx}}}{{\overset{.}{W}}_{comp}} \right)^{2}\left( \frac{\delta \; q_{{HEX},{Tx}}}{q_{{HEX},{Tx}}} \right)^{2}} + {\left( \frac{q_{{evap},{Tx}}}{{\overset{.}{W}}_{comp}} \right)^{2}\left( \frac{\delta \; q_{{Evap},{Tx}}}{q_{{Evap},{Tx}}} \right)^{2}}}} & (13)\end{matrix}$

where δx/x denotes the relative uncertainty associated with a randomvariable x. The relative uncertainties in {dot over (W)}_(comp) rangedfrom approximately 4% to 12%. For the COP, the uncertainty wascalculated from:

$\begin{matrix}{\frac{\delta \; {COP}}{COP} = \sqrt{\left( \frac{\delta \; {\overset{.}{W}}_{comp}}{{\overset{.}{W}}_{comp}} \right)^{2} + \left( \frac{\delta \; q_{{HEX},{Tx}}}{q_{{HEX},{Tx}}} \right)^{2}}} & (14)\end{matrix}$

which ranged between approximately 4% and 13%. The uncertainty for theheat of reaction, ΔH, was estimated as ±17%, and consequently c wasestimated as ±60%.

Experiments were conducted to determine the ability of the system 10 tocool and maintain a prescribed temperature of the load coolant (fluid)75 under various conditions. Steady-state and transient responseexperiments were conducted in which the coolant inlet temperature wasvaried while the flow rate was held constant at approximately 10 L/min,as shown in Table 2. The results from the steady-state and transientexperiments are described below.

One concern with this configuration of the system 10 is the potentialfor a thermal capacity mismatch among the HEX reactor 25, the VCS 40,and the applied thermal load 45. As described above, the VCS 40increases the temperature at which heat is transferred from the appliedthermal load 45 to the HEX reactor 25 such that the required AC reactiontemperature for rapid decomposition is attained. However, during thisprocess, inherent inefficiencies associated with the VCS 40, in turn,may generate heat. The cooling capacity available with the HEX reactor25 has to accommodate heat from both the VCS 40 and the applied thermalload 45. Thus, preliminary experiments were conducted to establish theupper limits of the heat absorption rates and explore the systemcontrollability.

FIG. 6 shows the imposed heat load coolant 75 and HEX reactortemperatures and HEX reactor reactant pressures for a typicalcharacterization experiment to illustrate the sequence of operations.More particularly, FIG. 6 shows the temperatures associated withtransfer side 12 of the HEX reactor 25, slurry generator 15, andevaporator 42. The bottom portion of FIG. 6 depicts the pressures forthe inlet/outlet of the HEX reactor 25. For the first twenty seconds,the system 10 opens the unloader valve (not shown) of the vacuum pump 21to reduce the sudden pressure change in the HEX reactor 25. For the nexttwo minutes, the VCS 40 preconditions the HEX reactor 25 by increasingthe temperature to the reaction temperature. Rising temperatures in theHEX reactor 25 indicate that the VCS 40 is absorbing heat from thecoolant 75 and transferring it to the HEX reactor 25, but the HEXreactor 25 has not yet reached the selected reaction temperature tobegin AC injection and decomposition. Also, during the preconditioningperiod, the expansion valve (not shown) of the VCS 40 is active,controlling the amount of heat absorbed from the coolant loop. Atapproximately 260 seconds, the HEX reactor 25 reaches the reactiontemperature, and the slurry generator 15 begins injecting AC (e.g., heatabsorption material 30), and the HEX reactor 25 begins removing heat viaAC decomposition, which marks the beginning of the decomposition period.The transfer loop outlet temperature of the HEX reactor 25 quicklylevels off to approximately 65° C., which is maintained for the durationof the experiment. The transfer loop inlet temperature continues torise, but at a much lower rate, indicating the total heat absorbed inthe HEX reactor 25 is slightly less than the amount of heat transferredfrom the VCS 40, which is most likely due to thermal capacitance effectsprovided by the TES 50. During this period the coolant temperaturedifference is maintained at approximately 6° C. Thus, the coolanttemperature was well managed in the experiment. The rise in the reactantside 11 inlet pressure (e.g., at inlet 8) is due to the flow resistanceimposed by the isolator orifice 24 of the HEX reactor 25. Atapproximately 430 seconds, the AC supply has been depleted, and the HEXreactor 25 outlet temperature begins to rise, indicating a heatabsorption reduction in the system 10.

Experimentally, it is determined that the system 10 achieved steadystate during the decomposition phase once the load inlet temperature(e.g., at the inlet 8) of the HEX reactor 25 ceased to vary by more thanapproximately 0.01° C./s over a 60 second period. The average fluidtemperature and flow rates were taken over this interval, from which thesteady state heat transfer rate was computed using Eq. (7). Therepeatability of the calculated heat transfer among the three steadyload, uncontrolled capacity experiments (within 3%) confirms this steadystate criterion to be applicable. In FIG. 6, the steady stateapproximation is invoked over the time interval of approximately 370-430seconds. The temperature rise at the inlet 7 of the transfer loop 27 is0.013° C./s, while the average coolant temperature change is effectivelyzero. FIG. 6 shows an oscillatory behavior in the coolant outlettemperature. This behavior was a result of the controller 55, which“hunted” about the set point during operation. The coolant outlettemperature varied about the average (e.g., approximately 14.6° C.)±1.5°C. at a rate of 0.5 Hz.

For an imposed thermal load coolant inlet temperature initially atapproximately 21° C., the heat absorption rate averaged approximately4481 W, with an average outlet temperature of approximately 14.6° C.Table 3 shows the results of the steady load experiments withoutcapacity control.

TABLE 3 Results from initial steady-state characterization experimentswithout capacity control AC Coolant Slurry HEX Coolant Coolant flow flowinlet reactor heat outlet heat rate rate temp. rejection temp.absorption Test (g/s) (g/s) (° C.) rate (W) (° C.) rate (W) COP 1 1.78161.33 23.6 5699.5 14.5 4526.9 3.86 2 1.85 160.66 22.3 5813.8 14.64502.6 3.43 3 1.85 151.35 24.9 6186.0 14.8 4415.5 2.49

For the third experiment (e.g., Test 3 in Table 3) the VCS superheat wasreduced in an attempt to increase the coolant outlet temperature.However, the estimated COP of the VCS 40 was 30% lower than the othertwo experiments (e.g., Tests 1 and 2). This suggests that although theheat absorption capacity of the system 10 may be controlled usingsuperheat, the reduction in efficiency makes the method undesirable dueto the excess heat that must be removed by the HEX reactor 25.

The capacity (e.g., heat transfer capability) of the VCS 40 is a complexfunction of the operating conditions for each system component (e.g.,condenser 41, evaporator 42, compressor 43, etc.) and the thermal load45. In order to maintain these conditions, and by extension thetemperature in HEX reactor 25 to drive AC decomposition, as well as thetarget coolant temperature, the capacity of the VCS 40 should bebalanced with the imposed heat load. Thus, it is preferred to understandhow to properly control the heat absorption rate (e.g., capacity) of thesystem 10 in response to the thermal load 45 and coolant temperatureconstraints. Steady-state experiments were conducted to investigate howwell the capacity was controlled when subjected to different thermalloads 45 and coolant inlet/outlet temperature constraints. It wasinitially suspected that the overall capacity of the system 10 might becontrolled by the capacity of the VCS 40. Thus, an attempt to use a VCShot-gas bypass controller temperature set point to control the coolantoutlet temperature was made. Each experiment in this series used adifferent (e.g., fixed) coolant inlet temperature, and the coolant flowrate was constant for all experiments. The first two experiments had aheat absorption rate goal of approximately 4000 W (e.g., near fulldesign capacity), whereas the third experiment was intended todemonstrate the lower absorption rate of approximately 2000 W (e.g.,approximately half design capacity). VCS hot-gas bypass control wasagain used to accomplish the reduction. Table 4 lists the temperatures,flow rates, and heat absorbed in each of the controlled-capacityexperiments.

TABLE 4 Steady-state controlled-rate experiment performance targets andresults Process Coolant AC Coolant outlet Process Actual inlet flow ratetemp. outlet cooling temp. rate target target temp. rate Con- (° C.)(g/s) (W) (° C.) (° C.) (W) version COP 21 1.86 4000 15.5 16.1 3538.81.06 1.77 18 1.91 4000 12.5 13.0 3595.6 1.04 1.86 17 1.89 2000 14.0 14.51986.3 0.99 0.77

Each experiment achieved heat absorption rates and coolant outlettemperatures within approximately 12% and 4.5% of their targets,respectively, with the reduced-capacity experiment achieving heatabsorption within 3% of the target heat rejection rate. Conversionfraction estimates from Eq. (12) were approximately 100%.

Table 4 shows that the VCS hot gas bypass method sufficiently controlledthe capacity of the system 10. However, the COP listed for eachexperiment indicates that the hot-gas bypass control method introducessome inefficiency. The full-capacity experiments demonstrated total heatabsorption in the HEX reactor 25 to be approximately 50% higher than theheat removed from the coolant 75, the additional heat arising fromcompressor work input and inefficiencies expressed in Eq. (11). For thereduced-capacity experiment, the total heat absorption rate of the HEXreactor 25 was approximately 130% higher than the heat removed from thecoolant 75, which implies a significant thermal efficiency penalty isincurred when a HGBV (not shown) is employed to throttle the capacity ofthe VCS 40.

FIG. 7 shows the HEX reactor 25 transfer side 12 and inlet and outlettemperatures of the coolant supply 47 during the reduced cooling rateexperiment. The temperatures of the transfer side 12, and thus thetemperature in the VCS condenser 41, are shown to continue to fall afteran initial peak once AC decomposition has begun. To account for thisbehavior, the energy balance of the HEX reactor 25 utilized Eqs. (8) and(9). The outlet temperature of the condenser 41 dropped 6° C. from 300to 500 seconds, which suggests that the AC mass flow rate was higherthan desired and, as a consequence, the HEX reactor 25 removed more heatfrom the system 10 than desired. However, during the same period, thecoolant outlet temperature was effectively steady, in the range ofapproximately 15+1° C. Since the capacity of the HEX reactor 25 exceededthe imposed heat load for the third test, the HEX reactor 25 did notattain steady-state, according to the criterion described above, duringthe limited length of the decomposition period. This was indicated bythe continuously decreasing temperatures in the HEX reactor 25 shown inFIG. 6 following the start of the decomposition phase of the experiment.

In FIG. 7, the behaviors of the temperatures of the thermal load 45associated with the inlet/outlet 8, 9 of the HEX reactor 25, slurrygenerator 15, and evaporator 42 shows the “hunting” behavior observed inall steady-state experiments. However, since the temperature stabilitygoal was ±2° C., the performance of the system 10 was within designconstraints. Since the controller 55 utilizes a finite error signal togenerate a control output, a steady-state temperature offset in thecoolant outlet temperature of approximately +0.5° C. was observedconsistently.

In some thermal management systems, the thermal load 45 may be unsteady.Thus, experiments were conducted in which the thermal load 45 presentedto the system 10 varied in time. One set of experiments was performedwithout the TES 50 (e.g., apparatus 5 a shown in FIG. 4) to study thedynamic response of the system 10 without damping. A second set used theTES 50 to explore the thermal behavior of the system 10 with damping(e.g., apparatus 5 b shown in FIG. 5).

The unsteady load experiments without the TES 50 exhibited severecoolant temperature stability issues, wherein rapid cycling of the VCScompressor 43 caused the coolant outlet temperature to vary betweenapproximately 8 and 21° C., well outside the ±2° C. tolerance. Based onthese experiments, the VCS 40 may not adequately control the coolanttemperature within the established limits while subjected to shortperiods of heating. Indeed, VCS control under highly dynamic loadingconditions, especially with strict temperature requirements, may beachieved using multi input/multi output control algorithms and/or systemarchitectures capable of dynamically controlling pressures, refrigerantcharge, or other parameters. Accordingly, the TES 50 may provide aviable remedy to this issue.

Therefore, an additional set of dynamic heat load experiments wereconducted with the TES 50 incorporated as an intermediate step betweenthe subsystem 46 and system 10, as shown in FIG. 5. The significantthermal capacitance of the TES 50 (during the phase transition)effectively decoupled the imposed heat load dynamics from the system 10,and consequently the evaporator 42 experienced what was comparable to asteady heat load regardless of pulsed load source input. FIG. 8 showsthe temperature and load heater power output histories of a TES50-integrated experiment. The outlet temperature of the coolant supply47 during operation of the system 10 averaged approximately 13° C. whilethe inlet coolant supply 47 temperature remained within approximately18±1° C.

FIG. 8 shows that the power of the thermal load heater 48 was cycledthrough four multi-pulse groups in this experiment. Three groupsproduced a nominal 300 kJ of heat each; the second burst was truncateddue to the TES 50 not being recharged before the burst occurred (thecoolant upper limit of 19.5° C. was exceeded). An initial period ofoperation of the VCS 40 is indicated by the rise in the temperatures ofthe HEX reactor 25 and VCS 40 from 10 to 60 seconds. This period wasused to pre-charge the TES 50 and preheat the HEX reactor 25.

A magnification of the first heater pulse group (i.e., Group 1) andrecharge cycle of the TES 50 of FIG. 8 is shown in FIG. 9. During theseventh heater pulse (e.g., at approximately 1290 seconds in FIG. 9),the TES 50 became fully discharged, as indicated by the increase in theslope in the outlet temperature of the coolant supply 47. This risetriggered the system 10, which started recharging the TES 50 and coolingthe coolant 75 approximately 20 seconds after discharge of the TES 50was detected. During the entire cycle the temperature of the coolantsupply 47 was remained within approximately 1° C. of the coolant setpoint of 18° C. The recharge period of the TES 50 lasted approximately100 seconds, and ended when the coolant outlet temperature of the TES 50dropped below the cut-off temperature set point of 16.5° C. of thesystem 10. The temperature of the coolant inlet remained withinapproximately 1° C. of the phase change temperature of 18° C. of the TES50. An approximately 10° C. difference between inlet and outlettemperatures of the coolant supply 47 occurred during the ‘on’ cycles ofthe system 10. The TES 50 provided a thermal buffer between the system10 and the coolant 75, thereby preventing the temperature swing in therecharge loop from propagating to the coolant loop. Table 5 shows theresults of transient heat absorption experiments with TES 50. The valuesrepresent average measurements over the steady operating periods ofmultiple TES 50 recharging cycles per experiment.

TABLE 5 Summary of unsteady heat load experiments with TES (Test 1) andwithout TES (Test 2) Duration Duty Avg. gload q recharge q_(HEX, Tx)Group # (s) Cycle (W) (W) (W) ε Unsteady load, with TES (Test 1) 1 87.80.81 3322 — — — 2 97.2 0.78 3153 2052 5504 0.73 3 96.5 0.81 3225 22184723 0.81 Unsteady load, with TES (Test 2) 1 96.5 0.78 3142 — — — 2 53.30.80 3230 2773 3028 0.74 3 96.5 0.78 3085 2340 3718 0.99 4 97.2 0.843349 2331 3985 0.99

FIG. 10, with reference to FIGS. 1 through 9, illustrates a systemdiagram of a heat sink 300 comprising the HEX reactor 25 to permit anexpendable heat absorption material 30 to absorb heat from a thermalload 45 at a temperature above 60° C. and a pressure below 3 kPa,wherein the expendable heat absorption material 30 endothermicallydecomposes. The vapor cycle system 40 is coupled to the HEX reactor 25and is configured to transfer heat from the thermal load 45. A firstcontroller 90 is provided to regulate a heat absorption into the vaporcycle system 40. In various embodiments, the first controller 90 may beany type of processor, microcontroller, application specific processor,application specific integrated circuit, or digital signal processor. Inan example, the first controller 90 may comprise thermocouples 91 todetect the temperature of the vapor cycle system 40. The thermocouples91 may be operatively connected to a microprocessing chip 93, whichcompares detected temperature readings from the thermocouples 91 topreprogrammed temperature levels stored in memory 94 and accessed by themicroprocessing chip 93. Accordingly, the microprocessing chip 93controls the amount of heat absorbed by the vapor cycle system 40 bytransmitting a control signal 78 to the vapor cycle system 40 tomaintain the temperature in the vapor cycle system 40 so that thetemperature is within a permitted range of the preprogrammed temperaturelevels. The heat sink 300 may also comprise a heat capacitor (e.g., TES50), which is operatively connected to the vapor cycle system 40,wherein the heat capacitor (e.g., TES 50) is configured to isolate theheat exchanger reactor 25 from thermal transients of the thermal load45. The expendable heat absorption material 30 may comprise anammoniated salt. The heat sink 300 may further comprise a carrier fluid85 mixed with the expendable heat absorption material 30 in a mixer 99,wherein the carrier fluid 85 comprises any of ethylene glycol andpropylene glycol. A second controller 92 is provided to regulate anamount of the expendable heat absorption material 30 into the HEXreactor 25. In various embodiments, the second controller 92 may be anytype of processor, microcontroller, application specific processor,application specific integrated circuit, or digital signal processor. Inan example, the second controller 92 may comprise a regulator 96, whichmay be operatively connected to the mixer 99, the HEX reactor 25, and amicroprocessing chip 97, which compares the amount of expendable heatabsorption material 30 in the HEX reactor 25 to preprogrammed desiredheat absorption material levels stored in memory 98 and accessed by themicroprocessing chip 97. Accordingly, the microprocessing chip 97transmits a signal 79 to the regulator 96 to control the amount ofexpendable heat absorption material 30 sent from the mixer 99 to the HEXreactor 25 so that the amount of expendable heat absorption material 30in the HEX reactor 25 is within a permitted range of the preprogrammedheat absorption levels.

FIG. 11, with reference to FIGS. 1 through 10, is a flow diagramillustrating a method 200 for managing heat transfer of a thermal load45, the method 200 comprising providing (202) a thermal managementsystem 10 comprising a heat exchanger reactor 25 containing anexpendable heat absorption material 30; applying (204) heat from athermal load 45 to the thermal management system 10 at a temperaturebetween approximately 18-30° C. using a thermal load coolant 75;transferring (206) heat from the thermal load 45 to a transfer fluid(e.g., carrier fluid 85) at a temperature above approximately 60° C.creating the heated (e.g., carrier fluid 85), wherein the heated (e.g.,carrier fluid 85) is introduced into the heat exchanger reactor 25;transferring (208) heat from the heated (e.g., carrier fluid 85) intothe expendable heat absorption material 30, wherein the expendable heatabsorption material 30 endothermically decomposes into a gaseousby-product 35; and maintaining (210) the thermal load coolant 75 withina specified temperature range independent of heat load dynamics.

In one example, the method 200 may comprise applying the heat from thethermal load 45 at a steady rate. In another example, the method maycomprise applying the heat from the thermal load 45 at an unsteady rate.The method 200 may comprise calculating an amount of heat transfer fromthe thermal load 45 to the expendable heat absorption material 30. Theexpendable heat absorption material 30 may comprise an ammoniated salt.The expendable heat absorption material 30 may comprise any of ammoniumcarbamate and ammonium bicarbonate. The method 200 may comprisesuspending the expendable heat absorption material 30 in a carrier fluid85 comprising any of ethylene glycol and propylene glycol. The method200 may comprise isolating the heat exchanger reactor 25 from thermaltransients of the thermal load 45. The method 200 may comprisecontrolling (e.g., using controller 55) a temperature of the thermalload 45 being applied to the thermal management system 10 toapproximately an ambient temperature surrounding the thermal load 45.

The embodiments herein provide a thermal management of a thermal load 45using the endothermic decomposition of an expendable heat absorptionmaterial 30, such as AC. Experimentally, the thermal load 45 may bespecified to be either unsteady pulsed or steady continuous operationfrom 0 to 4000 W at slightly sub-ambient temperature (e.g.,approximately 15-21° C.). Experiments were conducted which demonstratecontrolled heat absorption of approximately 1986 W and 3600 W withthermal load coolant return temperature targets of approximately 15.5°C., 12.5° C., and 14° C.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. It is to be understood thatthe phraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Those skilled in the art willrecognize that the embodiments herein can be practiced with modificationwithin the spirit and scope of the appended claims.

What is claimed is:
 1. A method for managing heat transfer of a thermalload, the method comprising: providing a thermal management systemcomprising a heat exchanger reactor containing an expendable heatabsorption material; applying heat from a thermal load to the thermalmanagement system at a temperature between approximately 18-30° C. usinga thermal load coolant; transferring heat from the thermal load to atransfer fluid at a temperature above approximately 60° C. creating aheated transfer fluid, wherein the heated transfer fluid is introducedinto the heat exchanger reactor; transferring heat from the heatedtransfer fluid into the expendable heat absorption material, wherein theexpendable heat absorption material endothermically decomposes into agaseous by-product; and maintaining the thermal load coolant within aspecified temperature range independent of heat load dynamics.
 2. Themethod of claim 1, comprising applying the heat from the thermal load ata steady rate.
 3. The method of claim 1, comprising applying the heatfrom the thermal load at an unsteady rate.
 4. The method of claim 1,comprising calculating an amount of heat transfer from the thermal loadto the expendable heat absorption material.
 5. The method of claim 1,wherein the expendable heat absorption material comprises an ammoniatedsalt.
 6. The method of claim 1, wherein the expendable heat absorptionmaterial comprises any of ammonium carbamate and ammonium bicarbonate.7. The method of claim 1, comprising suspending the expendable heatabsorption material in a carrier fluid comprising any of ethylene glycoland propylene glycol.
 8. The method of claim 1, comprising isolating theheat exchanger reactor from thermal transients of the thermal load. 9.The method of claim 1, comprising controlling a temperature of thethermal load to approximately an ambient temperature surrounding thethermal load.